Ultra high saturation moment soft magnetic thin film

ABSTRACT

A plated magnetic thin film of high saturation magnetization and low coercivity having the general form Co 100-a-b Fe a M b , where M can be Mo, Cr, W, Ni or Rh, which is suitable for use in magnetic recording heads that write on narrow trackwidth, high coercivity media. The plating method that produces the alloy includes four current application processes: direct current, pulsed current, pulse reversed current and conditioned pulse reversed current.

This is a Divisional Application of U.S. patent application Ser. No.09/859,363, filed on May 18, 2001, which has issued as U.S. Pat. No.6,776,891 on Aug. 17, 2004, and which is assigned to a common assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the formation of magnetic films foruse in fabricating recording heads suitable for writing on high densitymagnetic media. In particular the invention teaches a plating method forthe formation of an alloy of novel composition and particularlyadvantageous magnetic properties.

2. Description of the Related Art

Magnetic write heads must be capable of recording on magnetic media withrecording densities that will approach 100 Gb/in² by 2003. The highcoercive force necessary to record on such media, coupled with the highresolution required by the narrow trackwidth and recording density, willnecessitate the formation of write head material with high saturationmagnetization and low coercivity. Since modern write head manufacturingtechniques have turned to the thin film magnetic head as the structureof choice, a method must be found to form such thin films with asaturation magnetic moment preferably greater than 21 kG (kiloGauss) andlow coercivity, preferably less than 13 Oe. Materials having theseadvantageous magnetic properties have already been studied extensively.Osaka et al. (U. S. Pat. No. 6,063,512) provide a magnetic film of lowcoercivity (a “soft” film) having a Co—Ni—Fe ternary alloy compositionand trace amounts of S and formed by a method of constant currentelectrodeposition. The film so provided is reported to have a saturationmagnetization, Bs, of between 1.5 T (Tesla) and 2.0 T (between 15 kG and20 kG) and a coercivity of less than 50 Oe (Oersteds). Further, Osaka etal. (U.S. Pat. No. 6,120,918) provide a magnetic film of high magneticmoment and low coercivity having a Co—Fe—Ni ternary alloy compositionwith mixed BCC (body centered cubic) and FCC (face centered cubic)crystal structure. Said film has a saturation magnetization, Bs, ofbetween 19 KG and 22 KG and a coercive force no greater than Hc=2.5 Oe.Although the soft film provided by Osaka has low coercivity, itssaturation magnetization is marginal for the high density recordingmedia envisioned. Other methods for forming magnetic films also sufferfrom the lack of sufficient magnetization. In this regard, Hasegawa (U.S. Pat. No. 6,124,047) provides a soft magnetic film of a Co-M-T-Ccomposition with advantageous resistivity and magnetostrictionproperties but having saturation magnetization of approximately 14 kG(1.4T). Suzuki et al. (U. S. Pat. No. 5,935,403) provides a method formanufacturing a magnetic thin film in which colloidal particles ofinsulating material are suspended within a plating bath comprising Fe,Ni and Co ions. The thin film thereby formed is characterized by asaturation magnetization of between 1.5 T and 1.8 T, which isinsufficient for the high density recording envisioned in the presentinvention.

Bozorth (“Ferromagnetism,” R. M. Bozorth, IEEE Press, New York, N.Y.1978, p. 190) describes an Fe₂Co alloy with a 24.3 kG maximum saturationmoment. This alloy, however, is conventionally produced by bulk meltingand high temperature thermal treatment, processes which are not suitablefor magnetic write head formation. In addition, as noted by Yun, et al.(“Magnetic Properties of RF Diode Sputtered Co_(x)Fe_(100-x) Alloy ThinFilms,” IEEE Trans. On Magnetics, 32(5), 9/1996, p 4535) this alloy alsohas an unacceptably high coercivity for application to write heads.

The particular method of electrodeposition applied to the formation ofmagnetic films also plays a role in achieving their advantageousproperties. In this respect, Asai et al. (U.S. Pat. No. 5,489,488) teachan electroplating process to form a soft magnetic multilayer film whosesuccessive layers are formed by alternating the current direction withinthe electrolyte. Liao et al. (U.S. Pat. No. 4,756,816) teach anelectroplating method using a low toxicity bath in which sodiumsaccharin acts as a stress relieving agent, boric acid acts as a pHbuffer and dodecyl sodium sulfate acts as a surfactant to eliminatepitting.

An effective method to reduce film coercivity is by promoting grainrefinement (smaller grain sizes). Grain refinement is generally achievedby enhancing nucleation or impeding grain growth duringelectrodeposition. As-deposited materials of mixed structure generallyhave smaller grain sizes because competition between structures promotesnucleation which, in turn, leads to more, but smaller, grains. The mixedFCC and BCC crystals of Co—Fe—Ni disclosed by Osaka et al. above is anexample of the use of multiple structures to reduce grain growth.Multiple, co-existing structures can also be formed by the addition ofminor amounts of elements such as Mo, Cr, W and Rh. FIG. 1 shows a phasediagram for a Co—Fe—Mo in which the Mo is present in approximately 5% byatomic weight. As can be seen, this small amount of Mo produces α, γ,and θ structures.

Another approach for reducing grain size is incorporating materialsthrough use of a dispersed metal oxide. The oxide interrupts graingrowth and thus enhances nucleation during electrodeposition. Oxides ofMo, W, Cr and Rh can be deposited from an aqueous solution under ananodic potential. FIG. 2 shows that MoO₃ can be deposited from anaqueous solution containing M₊₊₊ ions at anodic potential greater than0.2V and a pH less than 4. FIG. 2 also shows that that Mo can beoxidized to MoO₂ at a slightly cathodic potential. The MoO₂ can then befurther oxidized to MoO₃ at an anodic potential in an acidicenvironment.

Electroplating is an effective method for producing thin film magneticalloys. Co, Fe, Ni, Mo, Cr, W and Rh can be readily co-deposited from anaqueous solution of their salts by use of a cathodic current. The alloycontent can be adjusted by the solution concentration and currentdensity. The more concentrated element in the solution generallyproduces the more concentrated element in the alloy. Higher currentdensity favors the reduction of the element with the higher reductionpotential.

Adjusting plating parameters can fine tune some of the mechanical andmagnetic properties of the alloy film. For example, the addition ofsaccharin is known to reduce stress within the film (see Liao et al.,cited above.) Pulse and pulse reversal plating provides two potentialadvantages over direct current plating. One such advantage is thereduction of grain size by grain growth interruption with correspondinglowering of coercivity. Another advantage is improved micro-uniformity.The anodic period of the current allows the metal ion to be replenished,producing a uniformity of metal concentration across the topography ofthe film. This is particularly advantageous in plating applicationswherein the film is to be deposited in trenches with high aspect ratios,such as is the case when plating upper pole pieces of magnetic writehead elements.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a method for forming athin film magnetic alloy having a high saturation magnetic moment andlow coercivity.

A second object of this invention is to provide a method for forming athin film magnetic alloy having a high saturation magnetic moment andlow coercivity, wherein said film can be formed within trenches havinghigh aspect ratio.

A third object of this invention is to provide a method for forming athin film magnetic alloy having a high saturation magnetic moment andlow coercivity wherein said film is suitable for use in the fabricationof magnetic write heads for high density magnetic recording media.

In accord with the objects of this invention there is provided an alloyof Co—Fe—M, wherein the element M can be chosen from the groupconsisting of Mo, Cr, W, Ni or Rh and wherein said alloy has acomposition of the form Co_(100-a-b)Fe_(a)M_(b), wherein a is between 50and 80 and b is between 0 and 10 and wherein the as-deposited saturationmagnetic moment is greater than 20 kG and the easy-axis coercivity isless than 7 Oe.

Further in accord with the objects of this invention there is provided amethod of forming said alloy of the form Co_(100-a-b)Fe_(a)M_(b),wherein a is between 50 and 80 and b is between 0 and 10, by anelectroplating process using direct current, pulse, pulse reversal andconditioned pulse reversal and wherein said electroplating method issuitable for forming thin films of said alloy within trenches havinghigh aspect ratios such as is the case in upper pole pieces of magneticwrite head elements.

Further in accord with the objects of this invention is the applicationof pulse reversal and conditioned pulse reversal to the plating process,wherein there is an advantageous reduction of grain size by grain growthinterruption with corresponding lowering of coercivity of the as-platedfilm.

Still further in accord with the objects of this invention is theapplication of pulse reversal and conditioned pulse reversal to theplating process wherein there is obtained an advantageous improvement ofalloy micro-uniformity as the anode period of the current allows themetal ion to be replenished, producing a uniformity of metalconcentration across the topography of the film.

Also in accord with the objects of this invention there is provided analloy within the composition range Co_(100-a-b)Fe_(a)M_(b), wherein a isbetween 57 and 64 and wherein b is between 1.5 and 3, and wherein saidalloy has an as-deposited saturation magnetization moment greater than21 kG and an easy-axis coercivity of less than 7 Oe.

Still further in accord with the objects of this invention there isprovided a pulse reversal and conditioned pulse reversal electroplatingmethod for formation of the Co_(100-a-b)Fe_(a)M_(b) alloy wherein a isbetween 57 and 64 and b is between 1.5 and 3 and wherein saidelectroplating method is suitable for forming thin films of said alloywithin trenches having high aspect ratios.

Yet further in accord with the objects of this invention there isprovided a particular example of the Co_(100-a-b)Fe_(a)M_(b) alloywherein a is between 63 and 67 and b is between 0 and 0.5 and whereinthe as-deposited saturation magnetic moment of said alloy is greaterthan 23 kG and its easy-axis coercivity is less than 11 Oe.

Still further in accord with the objects of this invention there isprovided a direct current, pulse, pulse reversal and conditioned pulsereversal electroplating method for formation of theCo_(100-a-b)Fe_(a)M_(b) alloy wherein a is between 63 and 67 and b isbetween 0 and 0.5 and wherein said electroplating method is suitable forforming thin films of said alloy within trenches having high aspectratios.

Yet further in accord with the objects of this invention there isprovided an electroplating method for the formation of four elementfilm, a particular example being CoFe₆₇Ni₂Mo₃, which is formed by apulse reversal method.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention areunderstood within the context of the Description of the PreferredEmbodiment, as set forth below. The Description of the PreferredEmbodiment is understood within the context of the accompanying figures,wherein:

FIG. 1 is a CoFeMo equilibrium phase diagram at 20° C. and 1300° C.

FIG. 2 is an electrochemical equilibrium diagram for Molybdenum inaqueous solution.

FIG. 3 contains four graphical representations of anodic and cathodiccurrent plotted against time for different modes of plating provided bythe present invention.

FIG. 4 is a graphical representation of the as-deposited saturationmagnetization of plated CoFeMo alloy.

FIG. 5 is a graphical representation of the as deposited easy-axiscoercive force of of plated CoFeMo alloy within the same compositionalranges as specified in FIG. 4.

FIG. 6 is a graphical representation of B vs H loop traces (hysteresiscurves) for the easy and hard axes of a particular as-deposited CoFeMofilm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a method for forming a thin Co—Fe—M alloyplated magnetic film, which, as-deposited, has a saturationmagnetization of up to 24 kG and a minimum easy-axis coercivity of 6 Oe.Element M can be chosen from the group consisting of Mo, Ni, W, Cr orRh. The alloy has the general composition Co_(100-a-b)Fe_(a)M_(b),wherein, for optimality of the magnetic characteristics, the Co/Feweight ratio ranges from 50/50 to 20/80. Element M appears in the alloyat less than 10% by weight. The alloy contains trace amounts of oxidesof element M, such as MoO₂₋₃, CrO₁₋₅, RhO_(0.5-2), NiO_(1.33-2) orWO₂₋₃. The alloy is plated from a plating solution consisting of (a)cobalt sulfate or chloride salt; (b) ferrous sulfate or chloride salt;(c) supporting salt of sodium, potassium or ammonium chloride, sulfate,acetate, citrate, tartrate, or sulfamate; (d) pH buffering agent such asboric acid; (e) stress reducing agent such as sodium saccharin; (f)surfactant such as sodium lauryl sulfate, and (g) additional metal saltsuch as sodium or ammonium molybdenate to provide element M. An acidicplating environment is preferred in this application. The plating can beperformed with direct current, pulse, pulse reversal or conditionedpulse reversal.

Referring now to FIG. 3, there is shown a graphical representation ofcurrent vs. time waveforms for the various plating processes provided bythe present invention. FIG. 3( a) is a schematic depiction of a directcathodic current. FIG. 3( b) is a schematic depiction of a pulsedcathodic current, wherein the pulse width is indicated as “a” and thespacing between pulses is “b.” FIG. 3( c) is a schematic depiction of apulsed reversal current, wherein each cathodic pulse of width “a” isfollowed by an anodic pulse of width “b.” FIG. 3( d) is a schematicrepresentation of a conditioned pulse reversal current wherein thecathodic pulse is “conditioned” by a stepped reduction (or increase) ofpulse height of width “c” and the anodic current is conditioned by astepped increase in pulse height of width “d.”

Direct current plating produces minimal oxide formation in Co—Fe—M andCo—Fe alloys. Pulse plating shown in FIG. 3( b) deposits Co—Fe—M alloyduring cathodic current (current value “x” in figure) and oxidizeselement M when said current is reduced to zero. Pulse reversal platingshown in FIG. 3( c) is similar to pulse plating except that the briefanodic current-pulses of magnitude “y” enhances the oxidation of elementM. The conditioned cathodic pulse shown in FIG. 3( d), of step height“u” or “v” increases the content of element M in the alloy. Theconditioned anodic pulse of height “z” shown on the same graph favorsoxide formation. The magnitude of the cathodic pulse height depends onthe reduction potential of element M. For elements with low reductionpotential such as Ni, Mo and Rh, a conditioning current “u” that is lessthan “x” is favored. For W and Cr, whose reduction potential is higherthan the Co—Fe system, a higher current “v” is preferred. Table 1,below, summarizes the working range of current densities for Co—Fe—Mplating in accord with the objects of this invention.

TABLE 1 Working range of current density for direct current, pulse,pulse reversal and conditional pulse reversal Co—Fe-M plating. NotationSignificance Working Range x Cathodic current 5–30 mA/cm² y Anodiccurrent 0–30 mA/cm² u Cathodic conditioning  1–x mA/cm² current vCathodic conditioning x–60 mA/cm² current z Anodic conditioning currenty–50 mA/cm² a Cathodic pulse duration 10 ms–10 s b Anodic pulse duration 1 ms–1 s c Cathodic pulse conditioning  1 ms–9000 ms duration d Anodicpulse conditioning  1 ms–100 ms duration

The following two examples are disclosed as actual applications of themethod of the present invention as set forth within the description ofthe preferred embodiment herein.

EXAMPLE 1.

A high magnetic moment Co—Fe—Mo alloy with MoO_(x) is prepared by pulsereversal plating using solutions with the chemical concentrations setforth in Table 2 below and plating parameters set forth in Table 3below. The plating is performed in a paddle cell with a magnetic fieldof 1 kG. The substrate is 4.5″×4.5″ AlTiC with an Al₂O₃ undercoat andNiFe, CoNiFe, Cu, or Au sputtered underlayer. The thickness of saidunderlayer is between 0.07 μm (microns) and 0.1 μm. The plated filmthickness is controlled to be within the range of 0.7 μm and 1.3 μm.

TABLE 2 Co—Fe—Mo alloy plating solution. Component Concentration (g/l)CoSO₄7H₂O 15–30 FeSO₄7H₂O  8–50 NH₄Cl 12–20 H₃BO₃ 20–35 Na₂MoO₄2H₂O  0–0.5 C₇H₅NO₃S 0.5 C₁₂H₂₅NaO₄S 0.1 pH 2–4

TABLE 3 Typical plating parameters for Co—Fe—Mo alloy. OperatingParameters Notation Amount Cathodic current density x 15 mA/cm² Anodiccurrent density y 15 mA/cm² Cathodic duration a 1 s Anodic duration b 10ms Paddle speed 1 Hz

The plating process characterized by the chemical concentrations andphysical parameters in Table 2 and Table 3 respectively produces platedmaterials having high saturation moments. Referring to FIG. 4, there isseen a plot of plated film saturation moments over a wide range ofweight percentages of Co and Mo. Trace amounts of molybdenum oxide isformed during the anodic period. A wide range of materials in FIG. 4,consisting of alloys which are 55%–76% Fe by weight and greater than 0%Mo by weight, exhibit a saturation moment that is greater than 20 kG.Within this range two local maxima were found, indicated as “A” and “B”on the graph, wherein the saturation moment exceeded 23 kG. Thecomposition of the alloy within the “A” range consists of 63%–66.5% Feby weight and less than 0.2% Mo by weight. The highest saturation momentfound is 24 kG. The material composition corresponding to this value isCoFe_(64.5). Area B on the graph shows another local maximum wherein thesaturation moment is greater than 21 kG and wherein the alloycomposition is 58%–64% Fe by weight and 1.5%–3% Mo by weight. Thehighest saturation moment found within this region corresponds to analloy composition CoFe_(62.5)Mo_(2.4).

Referring next to FIG. 5, there is shown the as-plated easy-axiscoercivity of Co—Fe—Mo. Alloy composition within the range 58%–65% Fe byweight and greater than 1.6% Mo by weight exhibits as-deposited Hce thatis less than 7 Oe. The lowest coercivity found is 6 Oe at an alloycomposition CoFe_(62.5)Mo_(2.4), which is labeled C in the graph. Withinmeasurement tolerances, the alloy composition in C of FIG. 5, which isminimum coercivity, overlaps the alloy composition in B of FIG. 4, whichis maximum saturation moment. Combining the findings of these twofigures, two alloy compositions can be found that possess good magneticproperties meeting the objectives of this invention. For an applicationthat requires a maximum saturation moment, CoFe_(63-66.5)Mo_(<0.2)offers Ms>23 kG and Hce approximately 10 Oe. For an applicationrequiring balanced switching speed and saturation moment,CoFe₅₈₋₆₄Mo_(1.7-2.9) with Ms>21 kG and Hce<7 Oe is the alloy materialof choice.

Referring now to FIG. 6, there is shown a typical B—H loop trace ofas-plated CoFe_(62.5)Mo_(2.4) showing a saturation moment ofapproximately 22 kG and an easy-axis coercivity of approximately 5.9 Oe.

EXAMPLE 2.

Pulse reversal plating improves the as-plated coercivity. This exampleshows the advantage of pulse reversal over direct current plating. Theplating is performed under conditions similar to those given in Example1, except that samples #5 and #6 are plated in a solution containing anadditional 10 g/l of NiSO₄ 6H₂O. The as-deposited coercivities are givenin Table 4, below.

TABLE 4 Effects of additional elements and pulse reversal on as-plated,easy-axis coercivity. Sample# 1 2 3 4 5 6 Composition CoFe₆₅ CoFe₆₅CoFe₆₅Mo_(0.4) CoFe₆₅Mo_(0.4) CoFe₆₇Ni₂Mo₃ CoFe₆₇Ni₂Mo₃ Process DC PR DCPR DC PR Hce(Oe) 11 9 11 8 9 8

Pulse reversal (PR) plating results in reduction of as-depositedcoercivity (Hce) over direct current (DC) plating. This could be aresult of interrupted grain growth. The effectiveness of PR is morepronounced when when Mo is co-deposited with the Co—Fe alloy (samples#3–6). This may be a result of a co-deposition of MoO₂₋₃ causing afurther reduction in Hce. A similar effect was found in the Co—Fe—Ni—Mosystem.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than limiting of the present invention. Revisions andmodifications may be made to methods and materials employed in forming athin Co—Fe—M alloy plated magnetic film of general formCo_(100-a-b)Fe_(a)M_(b) having a high saturation moment and lowcoercivity, while still providing a method for forming such a thinCo—Fe—M alloy plated magnetic film of general formCo_(100-a-b)Fe_(a)M_(b) having a high saturation moment and lowcoercivity in accord with the spirit and scope of the present inventionas defined by the appended claims.

1. A plated magnetic pole structure for a magnetic write head capable ofwriting on high coercivity magnetic media at narrow trackwidths and atarea densities exceeding 100 GB/in² comprising: a substrate; a platedmagnetic pole structure formed on said substrate, said pole structurebeing formed of a plated thin film Co—Fe—M alloy of saturationmagnetization greater than 20 kG and easy-axis coercivity less than 13Oe, having the general compositional form Co_(100-a-b)Fe_(a)M_(b)wherein a is between 50 and 80, b is between 0 and 10 and M is anelement selected from the group consisting of Mo, Cr, W, Ni and Rh.
 2. Aplated magnetic pole structure for a magnetic write head capable ofwriting on high coercivity magnetic media at narrow trackwidths and atarea densities exceeding 100 GB/in² comprising: a substrate; a platedmagnetic pole structure formed on said substrate, said pole structurebeing formed of a plated thin film Co—Fe—M alloy of saturationmagnetization greater than 21 kG and easy-axis coercivity less than 7Oe, having the compositional form Co_(100-a-b)Fe_(a)M_(b) wherein a isbetween 57 and 74, b is between 1.5 and 3 and M is an element selectedfrom the group consisting of Mo, Cr, W, Ni and Rh.
 3. A plated magneticpole structure for a magnetic write head capable of writing on highcoercivity magnetic media at narrow trackwidths and at area densitiesexceeding 100 GB/in² comprising: a substrate; a plated magnetic polestructure formed on said substrate, said pole structure being formed ofa plated thin film Co—Fe—M alloy of saturation magnetization greaterthan 23 kG and easy-axis coercivity less than 11 Oe, having thecompositional form Co_(100-a-b)Fe_(a)M_(b) wherein a is between 63 and67, b is between 0 and 0.5 and M is an element selected from the groupconsisting of Mo, Cr, W, Ni and Rh.
 4. The plated magnetic polestructure of claim 1 where, in addition, there are contained traceamounts of oxides of element M in said Co—Fe—M alloy.
 5. The platedmagnetic pole structure of claim 2 where, in addition, there arecontained trace amounts of oxides of element M in said Co—Fe—M alloy. 6.The plated magnetic pole structure of claim 3 where, in addition, thereare contained trace amounts of oxides of element M in said Co—Fe—Malloy.
 7. A plated magnetic pole structure for a magnetic write headcapable of writing on high coercivity magnetic media at narrowtrackwidths and at area densities exceeding 100 GB/in² comprising: asubstrate; a plated magnetic pole structure formed on said substrate,said pole structure being formed of a plated thin film four componentalloy of the general form Co—Fe—Ni—Mo and a particular formCoFe₆₇Ni₂Mo₃.